JP3369468B2 - Inverter circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter circuit capable of stably lighting a cold-cathode fluorescent lamp (hereinafter referred to as a fluorescent lamp) as a load and improving accuracy of dimming by intermittent lighting.

[0002]

2. Description of the Related Art Fluorescent lamps lit by an inverter circuit are
For example, a conventional inverter circuit used as a backlight of a liquid crystal display device of a portable electronic device is shown in the circuit diagram of FIG. In FIG. 4, 1 is a piezoelectric transformer, 2
Is a single-winding flyback transformer, 3 is a fluorescent lamp, 4 is a buffer circuit, 5 is an error amplifier, 6 is a frequency modulator,
Reference numerals 7 and 8 denote power supply terminals, and Q1 is a switching transistor composed of an N-channel MOS transistor. The flyback transformer 2 and the switching transistor Q1 are connected in series, and the connection point P1 is connected to the input side of the piezoelectric transformer 1. The output side of the piezoelectric transformer 1 is connected to one end of the fluorescent lamp 3.

The other end of the fluorescent lamp 3 is grounded via a resistor R4 and connected to a rectifying circuit 9. The rectifier circuit 9 has the other end connected to one end of a variable resistor R3 which is grounded. The reference voltage source E1 is connected to the non-inverting input terminal of the error amplifier 5,
The variable resistor R3 is connected to the inverting input terminal. The output side of the error amplifier 5 is connected to the control terminal 11 of the frequency modulator 6 via the resistor R5, and the output side of the frequency modulator 6 is connected to the input side of the buffer circuit 4. A time constant circuit formed by a series circuit of a resistor R6 and a capacitor C3 is connected to the control terminal 11. The output side of the buffer circuit 4 is connected to the gate of the transistor Q1. The error amplifier 5, the frequency modulator 6, and the buffer circuit 4 are driven by the pulse voltage PS applied to the ON / OFF terminal 10 of the inverter circuit, and the drive voltage is supplied from the power supply terminal 7 on the high potential side through the switch 20 formed of, for example, a PNP transistor. To be done. The power supply terminal 7 on the high potential side is also connected to one end of the flyback transformer 2, and the power supply terminal 8 on the low potential side is grounded.

The resistor R4, the rectifier circuit 9 and the variable resistor R3 form a current detection circuit for the fluorescent lamp 3 and turn it into a voltage. Then, the voltage corresponding to the current of the fluorescent lamp 3 set by the variable resistor R3 and the voltage of the reference voltage source E1 are compared by the error amplifier 5. Further, the drain of the transistor Q1 and one end which is not connected to the connection point P1 on the input side of the piezoelectric transformer 1 are grounded. As shown in the characteristic diagram of FIG. 5, in the piezoelectric transformer 1, the boost ratio of the output voltage to the input voltage varies depending on the input frequency, and the output changes. At a frequency higher than the frequency f ₀ at which the boost ratio becomes maximum, the output decreases as the input frequency increases. Normally, the output of the piezoelectric transformer 1 is controlled in the vicinity of the frequency f ₁ which is slightly higher than the frequency f ₀ at which the step-up ratio becomes maximum. The frequency modulator 6 has an oscillation circuit inside, and as shown in the explanatory view of FIG. 6, the oscillation frequency f _osc is changed by adjusting the outflow current i from the control terminal 11, Outflow current i as in (b)
The oscillation frequency f _osc becomes higher as the value increases.

Next, the operation of this inverter circuit will be described. There are two main operation modes of the inverter circuit. The first mode is to start the fluorescent lamp from a complete stop state to keep the fluorescent lamp in a stable and stable lighting state. There is a second mode in which the intermittent state and the stable lighting state are repeated by intermittently lighting.
In both the first mode and the second mode, the lighting state has a transitional lighting state time (hereinafter referred to as a lighting head) until a stable lighting state is obtained. Normally, the frequency modulator 6 operates in the above-mentioned frequency f in any mode.
Oscillation starts at a frequency slightly higher than ₁ , and after the fluorescent lamp 3 enters a stable lighting state, it almost settles at the oscillation frequency of frequency f ₁ . The inverter circuit supplies the drive voltage to the error amplifier 5, the frequency modulator 6, and the buffer circuit 4 by applying the pulse voltage PS to the on / off terminal 10 and turning on the switch 20, and enters the beginning of lighting. Pulse voltage P
The first mode is set when S goes low once only stepwise, and the second mode is set when high level and low level are repeated.

A relatively low resistance resistor R6 and capacitor C
The role of the time constant circuit composed of 3 is to operate in the first mode and the second mode.
It is to set the time of the initial lighting of the mode.
At the beginning of lighting, since the fluorescent lamp 3 is in a stable lighting state in both the first and second modes, the current of the fluorescent lamp is small, and the feedback signal from the resistor R3 in the detection circuit for detecting the current is Almost nothing, the output of the error amplifier 5 goes high. Therefore, no current flows out from the control terminal 11 of the frequency modulator 6 through the resistor R5. However, the capacitor C3 in series with the resistor R6 connected to the control terminal 11
Since the electric charge of is initially zero, the resistance R from the control terminal 11
A relatively large outflow current flows through 6, and the oscillation frequency f _osc of the frequency modulator 6 becomes high. As the charge of the capacitor C3 is accumulated, the outflow current decreases and the oscillation frequency _fosc decreases. During this period, the output voltage of the error amplifier 5 decreases due to the detection voltage of the fluorescent lamp current that gradually increases, and the outflow current from the control terminal 11 through the resistor R5 is replaced with the outflow current to the capacitor C3, and the oscillation frequency f _osc of the frequency modulator 6 is changed. Will settle at approximately frequency f ₁ .

The output applied from the piezoelectric transformer 1 to the fluorescent lamp 3 becomes constant, and a stable lighting state is achieved. In the second mode, the fluorescent lamp 3
Is intermittently lit by the repetition of the high level and the low level of the pulse voltage PS, so that the light is dimmed to a predetermined brightness. The dimming level is set by adjusting the duty ratio, which is the ratio of the high level and the low level of the pulse voltage PS applied to the on / off terminal 10.
In the circuit of FIG. 4, the time at the beginning of lighting in the first and second modes is set by the time constant circuit of the resistor R6 and the capacitor C3 and becomes the same.

FIG. 7 and FIG. 8 are waveform diagrams of each operation mode. FIG. 7 is a waveform diagram showing the fluorescent lamp current and the input voltage of the piezoelectric transformer in the first mode of the circuit of FIG.
They are shown separately in (a) and FIG. 7 (b). The input voltage of the piezoelectric transformer is the voltage at the connection point P1. The horizontal axis is a common time axis. In the first mode, the step circuit low level of the pulse voltage PS is applied to the on / off terminal ₁₀ at time t ₁₀ , so that the inverter circuit is activated,
Fluorescent lamp 3 turns on. The frequency modulator 6 has a resistor R
Oscillation is performed at a frequency set by the time constant circuit composed of 6 and the capacitor 3, and the oscillation frequency f _osc decreases as the charge is accumulated in the capacitor C3. During this period, the oscillation output of the frequency modulator 6 is applied to the gate of the switching transistor Q1 via the buffer circuit 4. And
By applying the flyback voltage having the same frequency as the oscillation frequency _fosc as the input voltage of the piezoelectric transformer 1,
The output of the piezoelectric transformer 1 gradually increases. Then, lighting the first head to the time t ₁₁ is in a stable lighting state at the end. The oscillation frequency f _osc of the frequency modulator 6 in the stable lighting state after the time t ₁₁ settles to the frequency f ₁ almost.

In a stable lighting state, a voltage corresponding to the current of the fluorescent lamp 3 is applied as a feedback signal to the control terminal 11 of the frequency modulator 6 via the error amplifier 5. And
For example, if the current of the fluorescent lamp 3 increases and its amplitude changes for some reason, the voltage of the feedback signal at the inverting input terminal of the error amplifier 5 increases, and it passes through the resistor R5 to the control terminal 1
The output of the error amplifier 5 added to 1 becomes low. As a result, the outflow current through the resistor R5 of the frequency modulator 6 increases, the oscillation frequency f _osc temporarily increases, and the output of the piezoelectric transformer 1 decreases. In this way, the frequency of the fluorescent lamp current in the stable lighting state is almost settled to the frequency f ₁ of the frequency modulator 6, and a series of feedback control in which the amplitude is kept constant is performed. In this feedback control, since the capacitor C3 of the time constant circuit of the resistor R6 and the capacitor C3 has a constant voltage, it does not contribute to the oscillation of the frequency modulator 6. The linear part of the fluorescent lamp current and the input voltage of the piezoelectric transformer in FIG. 7 is the envelope of the oscillating current and the oscillating voltage due to the oscillation of the frequency modulator 6. Only a part of the oscillating current and the oscillating voltage is shown in the figure, and it is shown that the frequency is high at the beginning of lighting and low at a stable lighting state. This also applies to FIG.

FIG. 8 is a waveform diagram showing the fluorescent lamp current and the input voltage of the piezoelectric transformer in the second mode of FIG. 4, which are shown separately in FIGS. 8 (a) and 8 (b). The horizontal axis is a common time axis. Lighting is between time t ₁₃ from the time t ₁₂ the first head, between the time t ₁₄ from the time t ₁₃ is a stable lighting state, time t
It is in a disconnected state in which the light is not turned on from ₁₄ to time t ₁₅ . Time t ₁₂
At the beginning of lighting, the frequency modulator 6 passes through the resistor R6 to the capacitor C3 discharged in the disconnection state, and the control terminal 11
Current oscillates to oscillate at a high frequency, and as the charge is accumulated in the capacitor C3, the oscillation frequency f _osc
Goes down. Then, the output of the piezoelectric transformer 1 increases, and a stable lighting state is achieved at time t ₁₃ . The oscillation frequency f _osc of the frequency modulator 6 in the stable lighting state after the time t ₁₃ settles to almost the frequency f ₁ . Then, the conducted discharge of the capacitor C3 is again disengaged state from the time t _14, toward the time t ₁₅ of the next lit first head. The time from the time t ₁₂ to the time t ₁₃ of the lighting head is from the time t ₁₀ to the time t in the first mode.
Same as the time between ₁₁ .

However, in such two kinds of operation modes of the inverter circuit, the first mode for starting the inverter circuit from a complete stop and keeping the fluorescent lamp 3 in a stable lighting state and the first mode for dimming. The request content for the lighting head is different between the 2nd mode. In the first mode, since the lighting is performed from the completely extinguished state, it is better to lengthen the time of the initial lighting head to the stable lighting state in order to stabilize the gas inside the fluorescent lamp 3. For this reason, the oscillation frequency f _osc of the frequency modulator 6 is slowly and stably changed to the frequency f in the lighting state.
It is desirable to lower it to ₁ . In the second mode, since the disconnection state and the lighting state have already been repeated in a short time, it takes almost no time to stabilize the gas inside the fluorescent lamp 3. Moreover, in the second mode, in order to perform dimming proportional to the duty ratio, which is the ratio of the high level and the low level of the pulse voltage PS, the time at the beginning of lighting is shortened and the low level time of the pulse voltage PS is reduced. Since it is desirable that almost all of them are in the stable lighting state, it is necessary to set the frequency f ₁ of the stable lighting state in a shorter time than in the first mode.

If the frequency of the pulse voltage PS applied to the terminal 10 is 100 Hz, one cycle is 10 ms, and considering the dimming when the duty ratio is 50%, the low level time of the pulse voltage PS is about 5 ms. It is desirable to set the initial lighting head for practical use within 1 ms, which is smaller than 5 ms. On the other hand, in the first mode, in order to stabilize the internal gas, it is desirable that the initial lighting head until the stable lighting state is set to 10 ms or more. As described above, in the first mode and the second mode, it is necessary to make the lighting start time different, but the number of time constant circuits connected to the control terminal 11 of the frequency modulator 6 is one. As a result, it was not possible to set the optimal lighting head time for each mode. Therefore, the time at the beginning of lighting has to be set to a time of about 3 ms, which can barely be commonly used for the purposes of both the first mode and the second mode. The time at the beginning of lighting is not a satisfactory result because it is a product of compromise, and a lighting error occurs in which a stable lighting state cannot be obtained in the first mode, or the dimming accuracy in the second mode is poor. The inconvenience that the range of dimming was insufficient often occurred.

[0013]

The problems of the present invention are as follows.
It is possible to set the lighting start time in the mode and the second mode separately, eliminate the mistaken lighting of the fluorescent lamp in the first mode, and set the dimming accuracy and dimming range for the second mode. It is to provide an inverter circuit that can be improved.

[0014]

An inverter circuit of the present invention is an inverter circuit for lighting a cold cathode fluorescent lamp by using a piezoelectric transformer, and a flyback transformer connected in series with a switching transistor and a flyback voltage of the transformer. It has a piezoelectric transformer that adds an input and an output to the cold cathode fluorescent lamp, and an oscillation frequency is adjusted by a feedback signal corresponding to the current of the cold cathode fluorescent lamp, and the oscillation frequency has a frequency modulator that turns on and off the switching transistor through a bacher. There is. In the frequency modulator, the oscillation frequency is adjusted by controlling the current flowing out from the control terminal, a feedback signal is added to the control terminal, and a plurality of time constant circuits having different time constants are connected in parallel. the mode activates a time constant circuit having a large time constant, the second mode Rukoto using a time constant circuit having a small time constant
Therefore, the time of the initial lighting portion is set and the cold cathode fluorescent lamp is dimmed.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The inverter circuit of the present invention has a control terminal for controlling the discharge current of a frequency modulator, and a time constant circuit having a large time constant for mainly setting the time of the beginning of lighting in the first mode. , A time constant circuit with a small time constant that mainly sets the time of the initial lighting period in the second mode is connected in parallel. As a result, in the first mode, the oscillation frequency f _osc of the frequency modulator 6 slowly drops to the frequency f ₁ in the stable lighting state, so that the time of the initial lighting period becomes long,
The oscillation frequency f in the second mode is higher than that in the first mode.
_{The osc} rapidly drops to the frequency f ₁ and the time at the beginning of lighting becomes short. Therefore, it is possible to set the optimum lighting start time in each mode.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an inverter circuit according to the present invention will be described below with reference to FIG. 1, which is a circuit diagram. The same parts as those in FIG. 1 are designated by the same reference numerals. Unlike FIG. 4, FIG. 1 has two time constant circuits connected in parallel to the control terminal 11 of the frequency modulator 6. The time constant circuit includes a series circuit of a resistor R1 and a capacitor C1, and the time constant circuit includes a series circuit of a resistor R2 and a capacitor C2. The resistors R1 and R2 have the same value, but the capacitance value of the capacitor C1 is larger than that of the capacitor C2, and the time constant of the series circuit of the resistors R1 and C1 is larger than that of the series circuit of the resistors R2 and C2. 20 times larger. In the embodiment, the time constant of the series circuit of the resistor R1 and the capacitor C1 is set to 2
0 ms, the time constant of the series circuit of the resistor R2 and the capacitor C2 is set to about 0.5 ms.

The operation of the above-described inverter circuit of the present invention will be described mainly on the part different from the conventional one. First, when the inverter circuit is activated in the first mode, the frequency modulator 6
The voltage of the control terminal 11 rises, the outflow current flows through the capacitors C1 and C2 having zero electric charge through the resistors R1 and R2, and the frequency modulator 6 has a frequency determined by the outflow current.
Oscillates. In this embodiment, the outflow current is the resistance R
1. The current flows through the resistor R2 at the same time for about 0.5 ms, but after that, it mainly flows through the resistor R1. Then, the capacitor C1 is gradually charged at the beginning of lighting for about 20 ms, and the outflow current from the control terminal 11 is also reduced, so that the oscillation frequency _fosc gradually decreases and a stable lighting state is _achieved . The oscillation frequency f _osc of the frequency modulator 6 in the stable lighting state is the frequency f ₁ ,
Feedback control is performed in the vicinity of this frequency f ₁ . That is,
The time at the beginning of lighting in the first mode is set mainly by the time constant of the series circuit of the resistor R1 and the capacitor C1. Since two time constant circuits are connected in parallel, the oscillation frequency f _osc of the first 0.5 ms at the beginning of lighting is higher than that of a single time constant circuit. Does not occur.

In the second mode, since the time constant of the time constant circuit composed of the resistor R1 and the capacitor C1 is large,
Even if the fluorescent lamp 3 does not light up and is disconnected, the capacitor C1
Almost no electric charge is discharged. In the embodiment, even when the pulse voltage PS is at the high level for 5 ms and the switch 20 is turned off to disconnect the inverter circuit, the capacitor C1 is discharged with the time constant of 20 ms, so that the terminal voltage does not drop. It is about 1/4, and is considered to hardly discharge. On the other hand, since the time constant of the series circuit including the resistor R2 and the capacitor C2 is small, the charge of the capacitor C2 is discharged in a short time. In the case of the embodiment, the resistor R2 and the capacitor C
Since the time constant of the serial circuit of 2 is 0.5 ms, the capacitor C2 is discharged if the pulse voltage PS has a high level time of 0.5 ms or more. At the next low level time of the pulse voltage PS, an outflow current flows from the control terminal 11 through the resistor R2 to the capacitor C2, and the frequency modulator 6 starts oscillating at a frequency higher than the frequency f ₁ determined by this outflow current. To do. Then, the oscillation frequency becomes the frequency f ₁ in a stable lighting state at about 0.5 ms which is the time constant of the series circuit of the resistor R2 and the capacitor C2. Even after 0.5 ms has passed, a slight charging current of the capacitor C1 flows out from the control terminal 11 due to the time constant of the resistor R1 and the capacitor C1, but it is corrected by the feedback control and the frequency modulator 6 is in a lighting state of almost the frequency f ₁ . Oscillate. As described above, the lighting start time in the second mode is a short time of about 0.5 ms, and the lighting start time is set mainly by the time constant of the series circuit of the resistor R2 and the capacitor C2.

2 and 3 are waveform charts of the respective operation modes. FIG. 2 is a waveform diagram showing the fluorescent lamp current and the input voltage of the piezoelectric transformer in the first mode of the circuit of FIG. 1, which are shown separately in FIGS. 2 (a) and 2 (b). At time t ₁ , a low level of the pulse voltage PS is applied to the terminal 10, the inverter circuit is activated, and the fluorescent lamp 3 enters the lighting head. As described above, the frequency modulator 6 mainly oscillates at a frequency set by the time constant circuit of the resistor R1 and the capacitor C1. Then, the oscillation frequency f gradually
_osc goes down. During this period, the oscillation output of the frequency modulator 6 is applied to the gate of the switching transistor Q1 via the buffer circuit 4. Then, a flyback voltage having the same frequency as the oscillation frequency f _osc is applied as an input voltage to the piezoelectric transformer 1, so that the output of the piezoelectric transformer 1 increases. Then, at time t ₂ , the lighting state becomes stable. The oscillation frequency f _osc of the frequency modulator 6 in the stable lighting state after the time t ₂ settles to the frequency f ₁ almost. Time t
_The time at the beginning of lighting from ₁ to time t ₂ is about 20 ms. The feedback operation in the stable lighting state is the same as in the case of FIG.

FIG. 3 is a waveform diagram showing the fluorescent lamp current and the input voltage of the piezoelectric transformer in the second mode of the circuit of FIG. 1, which are shown separately in FIGS. 3 (a) and 3 (b). . At the time t ₃ , the lighting head enters the lighting head. Frequency modulator 6
Oscillates at an oscillation frequency _fosc mainly set by the time constant of the series circuit of the resistor R2 and the capacitor C2. Then, the oscillation frequency f _osc drops rapidly compared to the first mode, the stable lighting state at time t _4. This time t
_The beginning of lighting from ₃ to time t ₄ is about 0.5 ms. The frequency of the pulse voltage PS of the terminal 10 is 100 Hz,
When the duty ratio is 50%, the stable lighting state time is ideally 5 ms, but the actual stable lighting state time is 4.5 ms. In the conventional inverter circuit shown in FIG. 4, since the lighting head in the second operation mode under the same conditions is 3 ms, the stable lighting time is only 2 m.
It is about s. In the present invention, as described above, the time of the stable lighting state between the low levels of the pulse voltage PS can be lengthened,
You can get closer to your ideal. That is, the pulse voltage P
Since a stable lighting state and disconnection state that are substantially proportional to the duty ratio of S can be obtained, the variable range of dimming can be widened and its accuracy can be improved. Although the flyback transformer used in the embodiment is a single-winding transformer, a transformer in which the primary side and the secondary side are insulated may be used. Also, it is not necessary to limit the number of switching transistors and flyback transformers to one, and for example, a push-pull circuit may be formed using two sets of switching transistors and flyback transformers, and the output thereof may be applied to the piezoelectric transformer. You can also The present invention has a wide application range in an inverter circuit in which the output of a piezoelectric transformer is controlled by changing the input frequency of the piezoelectric transformer using a frequency modulator, and all of these are outside the scope of the present invention. is not.

[0021]

As described above, the inverter circuit of the present invention has the control terminal for controlling the outflow current of the frequency modulator,
Optimal for each mode by connecting in parallel a time constant circuit that mainly sets the time of the lighting head in the first operation mode and a time constant circuit that mainly sets the time of the lighting head in the second operation mode. You can set the initial lighting time. In the first mode, the lighting error can be eliminated by lengthening the time of the initial lighting portion. In the second mode, the time of the stable lighting state can be extended to almost all of the lighting state by shortening the time of the initial lighting part,
Since the time of the level of the pulse voltage for lighting and the time of a stable lighting state can be made substantially the same, the variable range of dimming can be widened, and at the same time the dimming accuracy can be improved. These are extremely practical advantages that conventional inverter circuits do not have.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an inverter circuit of the present invention.

FIG. 2 is a waveform diagram of the circuit of FIG.

FIG. 3 is another waveform diagram of FIG.

FIG. 4 is a circuit diagram of a conventional inverter circuit.

FIG. 5 is a characteristic diagram of a piezoelectric transformer.

FIG. 6 is an explanatory diagram of a frequency modulator.

FIG. 7 is a waveform diagram of the circuit of FIG.

8 is another waveform diagram of the circuit of FIG.

[Explanation of symbols]

1 Piezoelectric transformer 3 fluorescent lights 6 Frequency modulator 11 Control terminal

Claims (2)

(57) [Claims] 1. A flyback transformer connected in series with a switching transistor in an inverter circuit for lighting a cold cathode fluorescent lamp by using a piezoelectric transformer, and a piezoelectric transformer that receives a flyback voltage of the transformer as an input and adds an output to the cold cathode fluorescent lamp. , Has a frequency modulator whose oscillation frequency is adjusted by a feedback signal corresponding to the current of the cold cathode fluorescent lamp, and turns on and off the switching transistor at the oscillation frequency, and the frequency modulator controls the outflow current. Has a control terminal whose oscillation frequency is adjusted, a feedback signal is applied to the control terminal, and a plurality of time constant circuits having different time constants are connected in parallel. operates the time constant circuit large, the second mode Rukoto using a time constant circuit having a small time constant
Thus, an inverter circuit for a cold cathode fluorescent lamp is characterized in that the time of the lighting head is set to adjust the cold cathode fluorescent lamp. 2. The inverter circuit according to claim 1, wherein the time constant circuit is a series circuit of a resistor and a capacitor.